Re: Deadlock interaction between pthread_cond_check_need_init.c and pthread_cond_destroy.c

[Ross Johnson <rpj@ise.canberra.edu.au>]

Michael,

I've applied your patch and committed it to CVS. I'll write a test 
for it later.

Many thanks for this.
Ross

Michael Johnson wrote:
> When two different threads exist, and one is attempting to destroy a 
> condition variable while the other is attempting to initialize a 
> condition variable that was created with PTHREAD_COND_INITIALIZER, a 
> deadlock can occur:
> 
> Thread A Thread B
> Enters ptw32_cond_check_need_init
> 
> 
> Enters pthread_cond_destroy
> EnterCriticalSection(&ptw32_cond_test_init_lock)
> 
> (now holds ptw32_cond_test_init_lock)
> EnterCriticalSection(&ptw32_cond_list_lock)
> Enters pthread_cond_init
> Determines that condvar is static initialized
> EnterCriticalSection(&ptw32_cond_list_lock)
> EnterCriticalSection(&ptw32_cond_test_init_lock)
> (now waiting to enter ptw32_cond_list_lock)
> (now waiting to enter ptw32_cond_test_init_lock)
> deadlocked
> 
> 
> It appears from reading the code and from a patch that I made that the 
> following change to pthread_cond_destroy appears to fix this:
> |
> |
> 
>    |{|
>    |  pthread_cond_t cv;|
>    |  int result = 0, result1 = 0, result2 = 0;|
>    ||
>    |  /*|
>    |   * Assuming any race condition here is harmless.|
>    |   */|
>    |  if (cond == NULL|
>    |      || *cond == NULL)|
>    |    {|
>    |      return EINVAL;|
>    |    }|
>    ||
>    |  if (*cond != PTHREAD_COND_INITIALIZER)|
>    |    {|
>    |      EnterCriticalSection(&ptw32_cond_list_lock);|
>    ||
>    |      cv = *cond;|
>    ||
>    |      /*|
>    |       * Close the gate; this will synchronize this thread with|
>    |       * all already signaled waiters to let them retract their|
>    |       * waiter status - SEE NOTE 1 ABOVE!!!|
>    |       */|
>    |      if (sem_wait(&(cv->semBlockLock)) != 0)|
>    |        {|
>    |          return errno;|
>    |        }|
>    ||
>    |      /*|
>    |       * !TRY! lock mtxUnblockLock; try will detect busy condition|
>    |       * and will not cause a deadlock with respect to concurrent|
>    |       * signal/broadcast.|
>    |       */|
>    |      if ((result = pthread_mutex_trylock(&(cv->mtxUnblockLock)))
>    != 0)|
>    |        {|
>    |          (void) sem_post(&(cv->semBlockLock));|
>    |          return result;|
>    |        }|
>    ||
>    |      /*|
>    |       * Check whether cv is still busy (still has waiters)|
>    |       */|
>    |      if (cv->nWaitersBlocked > cv->nWaitersGone)|
>    |        {|
>    |          if (sem_post(&(cv->semBlockLock)) != 0)|
>    |            {|
>    |              result = errno;|
>    |            }|
>    |          result1 = pthread_mutex_unlock(&(cv->mtxUnblockLock));|
>    |          result2 = EBUSY;|
>    |        }|
>    |      else|
>    |        {|
>    |          /*|
>    |           * Now it is safe to destroy|
>    |           */|
>    |          *cond = NULL;|
>    ||
>    |          if (sem_destroy(&(cv->semBlockLock)) != 0)|
>    |            {|
>    |              result = errno;|
>    |            }|
>    |          if (sem_destroy(&(cv->semBlockQueue)) != 0)|
>    |            {|
>    |              result1 = errno;|
>    |            }|
>    |          if ((result2 =
>    pthread_mutex_unlock(&(cv->mtxUnblockLock))) == 0)|
>    |            {|
>    |              result2 = pthread_mutex_destroy(&(cv->mtxUnblockLock));|
>    |            }|
>    ||
>    |          /* Unlink the CV from the list */|
>    ||
>    |          if (ptw32_cond_list_head == cv)|
>    |            {|
>    |              ptw32_cond_list_head = cv->next;|
>    |            }|
>    |          else|
>    |            {|
>    |              cv->prev->next = cv->next;|
>    |            }|
>    ||
>    |          if (ptw32_cond_list_tail == cv)|
>    |            {|
>    |              ptw32_cond_list_tail = cv->prev;|
>    |            }|
>    |          else|
>    |            {|
>    |              cv->next->prev = cv->prev;|
>    |            }|
>    ||
>    |          (void) free(cv);|
>    |        }|
>    ||
>    |      LeaveCriticalSection(&ptw32_cond_list_lock);|
>    |    }|
>    |  else|
>    |    {|
>    |      /*|
>    |       * See notes in ptw32_cond_check_need_init() above also.|
>    |       */|
>    |      EnterCriticalSection(&ptw32_cond_test_init_lock);|
>    ||
>    |      /*|
>    |       * Check again.|
>    |       */|
>    |      if (*cond == PTHREAD_COND_INITIALIZER)|
>    |        {|
>    |          /*|
>    |           * This is all we need to do to destroy a statically|
>    |           * initialised cond that has not yet been used
>    (initialised).|
>    |           * If we get to here, another thread waiting to initialise|
>    |           * this cond will get an EINVAL. That's OK.|
>    |           */|
>    |          *cond = NULL;|
>    |        }|
>    |      else|
>    |        {|
>    |          /*|
>    |           * The cv has been initialised while we were waiting|
>    |           * so assume it's in use.|
>    |           */|
>    |          result = EBUSY;|
>    |        }|
>    ||
>    |      LeaveCriticalSection(&ptw32_cond_test_init_lock);|
>    |    }|
>    ||
>    |  return ((result != 0) ? result : ((result1 != 0) ? result1 :
>    result2));|
>    |}|
>